(1) Field of the Invention
The present invention pertains to an electrical circuit for operating warning lights that are commonly used on emergency and service vehicles. The warning lights incorporate halogen lamps and a rotating reflector. The reflector rotates or oscillates about the halogen lamp to give the appearance of a flashing light. The electrical circuit of the present invention provides a circuit for operating standard halogen lamps having a rating of 18-21 volt DC and a power rating of between 25-50 watts on vehicles having a 36-42 volt electrical system.
(2) Description of the Related Art
Since the invention of the rotating beacon in the 1940s, the vast majority of emergency and service vehicles have been protected by warning lights that use an incandescent lamp. In these systems, the lamp is given the appearance of flashing by a rotating reflector positioned in the warning light. The reflector is rotated around the lamp and produces a rotating, reflected arc of coverage and a level of warning within a specified zone around the emergency vehicle.
Although some emergency vehicles use other warning light systems, such as light emitting diodes (LEDs) and gaseous discharge tubes (strobe systems), the use of a halogen lamp and a rotating reflector is preferred and used on a majority of warning light systems because of its initial low cost, high efficiency, easy maintenance, and long life. With the advent of reasonably priced halogen incandescent lamps, the halogen cycle lamp, with its higher efficiency, has become a light source of choice, and these other systems have yet to match any of the advantages of the halogen lamp system.
Typically, standard halogen lamps are rated for operation between 12 and 24 volts and for operation between 25 and 50 watts. Current operational characteristics of the halogen lamps make them very adaptable for use on automobiles having electrical systems operating at between 12 and 14 volts. However, automobile manufacturers have recently announced the introduction of automobiles that will use a 42 volt electrical system. Thus, the change in the automotive voltage from 12 volts DC to 42 volts DC precludes the transfer of existing warning light systems to new emergency vehicles, as is commonly done today by many municipalities.
Additionally, the change in the electrical systems of automobiles to 42 volts presents other problems. At this time, no lamp manufacturer is able to manufacture a halogen lamp in the 25 to 50 watt range that can operate at 42 volts in automotive service. In automotive systems that use halogen lamps, there is a minimum diameter of tungsten filament that is required to maintain the halogen cycle. In order to develop a halogen lamp that operates at voltages that have been increased three-fold from 14 to 42 volts, for a given minimum practical filament diameter, the filament length must also increase three-fold. This consequently causes the wattage of the lamp to increase three-fold. While it is theoretically possible to replace three 50 watt elements with one 150 watt element, in reality, it is difficult to get the distribution of light (arc of coverage) that is desired in order to provide adequate levels of warning.
For instance, a typical light bar for an emergency vehicle is equipped with a system of halogen lamps that may be selectively illuminated to provide three different levels of warning. Each level of warning provides a different arc of coverage around the vehicle, and, generally speaking, two 50 watt lamps are provided in different sets at different positions on the light bar to generate the required arc of coverage and level of warning. In a low level warning condition, two 50 watt lamps are provided on the light bar to only cover the rear of the emergency vehicle. For an increased level of warning, the two 50 watt lamps are provided for rear only coverage and four 50 watt lamps are provided for additional coverage on the front, sides and rear of the vehicle. In a further increased level of warning, the two 50 watt lamps are provided for rear only coverage, the four 50 watt lamps are provided for additional coverage to the front, sides, and rear of the vehicle, and two additional 50 watt lamps are provided for added coverage to the front. As discussed earlier, increasing the voltage of the vehicle three-fold to 42 volts would also require increasing the wattage of the warning lamps three-fold to 150 watts. However, it is difficult to use a combination of 150 watt lamps, with each 150 watt lamp replacing three 50 watt lamps, and obtain this type of coverage. Moreover, the loss of a single 150 watt lamp causes a catastrophic loss of protection, in effect being equivalent to losing three 50 watt lamps. Thus, it is desired to continue the use of standard halogen lamps on current light bars and to adapt the light bar to operate with a 42 volt electrical system.
Halogen lamps in the 25 to 50 watt range have been successfully manufactured for operation in automotive service with voltages up to 24 volts. Although placing two 21 volt halogen lamps in series produces the required 42 volt voltage drop, placing two 21 volt lamps in series for operation at 42 volts presents problems that are unacceptable for operation in a light bar application. When two 21 volt lamps are placed in series, if one lamp fails, both are extinguished. Obviously, this presents an unsatisfactory condition in an apparatus designed to provide visible warning.
Additionally, connecting two halogen lamps in series produces a high rate of start-up failure for the lamps. The cold-resistance of the lamp filament of a typical halogen lamp is very small and varies substantially from lamp to lamp even between new lamps produced from the same manufacturer having identical ratings and capacity. For example, the cold resistance of the filament of a 14 volt, 50 watt halogen lamp may vary from 0.3 ohms to 0.7 ohms, although when lighted and heated, the filaments of identical lamps have virtually the same resistance of about 3.5 ohms. When a mix of new and old lamps from different manufacturers are measured, the cold resistance of the lamp filaments can vary even more. Taking into account the variations in the socket contact resistances and wire lengths in the light bar, the effective cold resistance of each lamp in the light bar greatly varies. This variation in cold resistance makes operating the halogen lamps in series problematic.
The inventor has observed that the filament life in a halogen lamp is a function of the voltage applied, the ambient temperature of the lamp""s glass envelope, and the heat sinking effect provided by the socket structure. The inventor has seen that at room temperature and voltages above 17 volts, the time to failure of the typical halogen lamp is very short. In a series connected arrangement, when the circuit is energized, each lamp will have a different initial voltage drop thereacross due to their different resistances. As the filament heats to incandescence, the voltages across the lamps will equalize. However, when the lamps are connected in series and the circuit is initially energized, the mismatch in cold resistances of the filaments often creates a high voltage drop across one of the two lamps. This increases the failure rate of the lamp. In order to limit failure when lamps are connected in series, the lamp with the lower resistance filament must heat quickly to gain resistance so as to prevent failure of the higher resistance filament of the other lamp with which it is connected in series. Again, this condition is unacceptable in an apparatus designed to provide visible warning.
Therefore, what is needed is a light bar that allows the use of standard halogen lamps having a voltage rating of between 18 and 21 volts and a power rating of between 25 to 50 watts on an emergency vehicle equipped with a 36-42 volt DC electrical system. Such a light bar would be provided with an electrical circuit to allow use of standard halogen lamps in the standard light bar when it is installed on a new emergency vehicle equipped with the 36-42 volt electrical operating system. Such a circuit would allow the use of halogen lamps having different cold resistances. Such a circuit would protect mismatched lamps from failure during start up. Such a circuit would maintain the required level of warning in case of failure of a lamp. Such a circuit would be easily retrofitted onto existing light bars to permit the light bar to be installed on new automobiles equipped with the 36-42 volt DC electrical system.
The electrical circuit of the present invention enables current users of halogen lamp warning lights to use a standard halogen lamp having a voltage rating of between 18 and 21 volts DC in a light bar. The electrical circuit is installed in light bars that are to be used on emergency vehicles equipped with the 36-42 volt electrical systems. The circuit basically enables the series connection of 18-21 volt lamps of the light bar, thereby adapting the light bar for use on a vehicle with a 36-42 volt electrical system. The electrical circuit of the present invention prevents start up failure of the halogen lamps and maintains the required level of warning in case of failure of a lamp.
The circuit of the present invention provides a first lamp paired with a first resistor, and a second lamp paired with a second resistor. The circuit also includes means for switching the circuit between a start up circuit where the series connected first lamp and first resistor are connected in parallel with the series connected second lamp and second resistor, and an operation circuit where the first lamp is disconnected from the first resistor and the second lamp is disconnected from the second resistor and the first lamp is connected in series with the second lamp. The switching means employs a time delay switch that is operable to switch the circuit from the start up circuit to the operational circuit after the first and second lamps reach incandescence. Thus, in the start-up circuit, the circuit allows the lamps to become illuminated through their respective resistors and achieve a uniform and relatively high illuminated resistance before the lamps are switched into a series connection in the operation circuit.
In another aspect of the invention, the switching means switches the circuit from the operation circuit to the start up circuit when the series connection between the first and second lamps is opened by either a lamp failure or a lamp being removed from the circuit. Thus, the electric circuit of the present invention maintains one of the operational lamps illuminated by reconnecting the operational lamp through its resistor in the start-up circuit. In this way, the desired level of warning may be maintained in the event of a failure of one of the two lamps. Preferably, the switching means allows the circuit to be operated as required with the one operational lamp until the other failed lamp is replaced.
The electric circuit of the present invention uses a resistor having a relatively high cold resistance in series with each lamp to bring the lamp to incandescence. The circuit of the present invention connects two lamps in series only after each lamp has reached incandescence, thus preventing start up failure of the lamps from excessive voltage at initial circuit energization. The circuit of the present invention detects the failure of either of the lamps operating in series and instantly reconnects the lamps to their respective start up resistors, thus allowing the operational lamp to continue to function with minimum degradation of the required level of warning. The circuit of the present invention may be operated and repeatedly turned on and off with the operational lamp continuing to operate at full brightness before the failed lamp is replaced. Thus, the electrical circuit of the present invention allows current 14 volt light bars to be retrofitted to emergency vehicles equipped with the 36-42 volts DC operating system with the 14 volt lamps replaced with pairs of 18-21 volt halogen lamps.